Depends on the lens really, the SWD ones are generally good as it's mechanical however the focus by wire ones are trying (focusing different amounts depending on speed so turning it slowly is really really slow, but slightly faster is too fast).

On an E-M1 the AF is perfectly usable, also some lenses such as the 14-54mm mkII are CDAF enabled.

Personally I couldn't manually focus using a focus by wire lens, it's less accurate, unpredictable, and slower than 'slow' autofocus.

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